Fluorescent materials containing a rare earth element, such as Eu (europium) or Tb (terbium), as a fluorescer, are known. Such fluorescent materials are produced by applying a powdery fluorescent substance containing a rare earth element to a support; forming a coating film containing a fluorescent substance on a support by sol-gel method; or like process (Japanese Unexamined Patent Publication No. 2001-270733; M. Nogami and Y. Abe, “Enhanced emission from Eu2+ ions in sol-gel derived Al2O3—SiO2 glasses”, Appl. Phys. Lett., 69(25) 3776 (1996), American Institute of Physic; M. Nogami, “Fluorescence properties of Eu-doped GeO2—SiO2 glass heated under an H2 atmosphere”, J. Luminescence, 92,329 (2001), Elsevier Science; etc.).
Fluorescent materials obtained by applying a powdery fluorescent substance are already being practically used, for lamps, cathode-ray tubes, etc. However, in most of such materials, the fluorescent substance is applied to the surface of the support, and thus such materials are opaque and generate merely superficial fluorescence.
Generally, fluorescent materials for high intensity lamps, displays, adjustment of short wavelength lasers or other purposes are required to be transparent and capable of being bulk-molded. Materials containing a stable oxide glass as a matrix may be conceived of as such fluorescent materials. In oxide glasses, however, the rare earth element or the like as a luminescence center generally binds strongly to the oxide glass matrix, and thus nonradiative transitions are likely to occur, making it difficult to obtain strong luminescence.
Glasses containing 2 to 15 mol % of Tb or Eu, calculated on an oxide basis, are known as oxide glasses with a relatively strong luminescence (Japanese Unexamined Patent Publication No. 1998-167755). Such glasses are excellent in heat resistance, chemical durability, mechanical strength, etc., but have a drawback in that they extremely expensive because of the high content of rare earth elements.
Fluorescent glasses prepared using a fluoride glass or oxyfluoride glass are also known (Japanese Unexamined Patent Publications No. 1996-133780, No. 1997-202642, etc.). However, such fluorescent glasses are poor with respect to heat resistance, chemical durability, etc., and have insufficient mechanical strength. It is therefore difficult to form such fluorescent glasses into large glass plates and to use them in the air, and especially outdoors, for long periods. These glasses have many other problems, such as the harmful effects of fluoride on the environment.
Other known production processes for luminescent glasses include doping of porous oxide glasses with ions or semiconductor fine particles (U.S. Pat. No. 6,211,526; A. L. Huston, B. L. Justus and T. L. Johnson, “Fiber-optic-coupled, laser heated thermoluminescence dosimeter for remote radiation sensing”, Appl. Phys. Lett., 68 (24), 3377 (1996), American Institute of Physics; B. L. Justus and A. L. Huston, “Ultraviolet dosimetry using thermoluminescence of semiconductor-doped Vycor glass”, Appl. Phys. Lett., 67 (9), 1179 (1995), American Institute of Physics; B. L. Justus, A. L. Huston and T. L. Johnson, “Laser-heated radiation dosimetry using transparent thermoluminescent glass”, Appl. Phys. Lett., 68 (1), 1 (1996) American Institute of Physics, etc.). However, the luminescent glasses obtained by these processes are incapable of generating fluorescence with a sufficiently high luminescence intensity.